Method of vibrating metal during casting



Jan. 16, 1968 J. PETIT ET AL METHOD OF VIBRATING METAL DURING CASTING Filed Dec. 10, 1965 mastic? 5 CHAN/e N w In M 5 I mm 1 m mo 7 m ET M F M E LF W M A "mu w aw m n RS 0 j m h F M u m VARIABLE FREQUENCY United States Patent Ofitice 3,363,668 Patented Jan. 16, 1%68 Claims. (a. 164-71) This application is a continuation-in-part of our application Ser. No. 310,406, filed Sept. 20, 1963, now abandoned which was a continuation-in-part of our application Ser. No. 32,440, filed May 27, 1960, now abandoned.

The invention relates to a method of refining metals comprising simultaneously casting and refining an ingot of metal maintained in a vertical position in a mold, feeding the upper end of the ingot progressively with molten metal to be refined while imparting to the ingot mechanical vibrations having a frequency preferably Within a supersonic frequency range while the solid part of the ingot is constantly cooled so as to solidif the molten portion of metal, the solid portion of the ingot constantly acting as a support for the molten portion, and moving in a continuous manner the ingot and/ or the mold relative to each other so as to constantly maintain within the mold the portion of the ingot which is in course of solidification.

Various means have been described for introducing supersonic waves in metal ingots or bars. The most usual method comprises coupling the solid end of the bar to the vibrating core of a magnetostrictive vibrator, this coupling being a direct coupling or including an insertion between the bar and the core of rigidly interconnected rigid intermediate bars so that the vibrations are always transmitted exclusively through a solid structure.

This vibrator is of the general type having a core (or cores) of nickel, this core, or each of the cores, being excited by a coil fed with a current having an acoustic frequency or preferably a supersonic frequency, said core being if desired rigidly secured to a steel rod acting as an intermediate transmitting means.

This known ingot refining method avoids any risk of contamination of the metal by a different material. However, it has certain serious drawbacks for the following reasons.

In order to render the discussion more clear, it will first be recalled that the mechanical waves which are propagated in a bar, such as the ingot to be refined, and reflected at the free end of the bar constitute waves which can be termed reflected waves which interfere with the waves of the same frequency, which can be termed incident waves, coming from the other end of the bar moved by the vibrator. There results an assembly of waves termed a standing wave system comprising a number of nodes and antinodes arranged in fixed positions and at fixed intervals along the bar, the distance between two consecutive nodes (or two antinodes) being equal to M2 in which is the length of the vibration and equals, as is well known V/ in which V is the velocity of propagation of the waves in the metal of the bar and f the frequency of these Waves.

The energy transmitted to the metal of the bar is maximum at the antinode position points whereas at the node position points this energy is theoretically nil.

In order that the method has maximum efficiency, it would be necessary that the molten portion of the treated ingot constantly coincide with a standing wave system antinode. This is impossible with the known method. In-

deed, in a bar excited at its natural resonance frequency, F0, an antinode is produced at the free end of this bar but this condition ceases to be satisfiied as soon as the length of the bar increases in the course of treatment since the natural frequency F0 varies and consequently ceases to be equal to the frequency F of the vibrator. The whole of the vibrating structure which is excited at a frequency other than its natural frequency is then the centre of a system of oscillations termed forced oscillations. Calculation shows that, if it is assumed that the frequency F is fixed and the value F0 is equal to that of F, the curve of the amplitude of the oscillations as a function of the ratio Fb/F has a maximum but that when F0 differs from F the value of the amplitude 0 very rapidly decreases so that the maximum of this curve has the very pointed appearance shown in FIG. 1. In other words, in such a system the resonance zone is very narrow and the efficiency of the method becomes consequently very low.

Theoretically, it would be possible to vary the whole of the standing wave system by continuously regulating the forced oscillation phase in such manner as to constantly maintain an antinode in coincidence with the free end of the ingot. In practice, this would amount to adjusting continuously the excitation frequency in accordance with the variation in the length of the treated ingot so as to ensure constantly that the excitation frequency F equals the frequency F0 whose value varies. However, such a regulation would be very difficult to obtain in practice owing to the narrow tuning range.

We have discovered that it is possible to obtain very satisfactory results with the method according to the present invention in which the value of the frequency F of the forced oscillation is regulated stepwise each time the length of the ingot to be treated has increased a certain value I and the frequency F is modulated in accordance with a periodic law so as to cause it to vary between two limits F and F on each side of the resonance value F0 of the ingot so that the value Fo/F is constantly Within a range corresponding to high values of oscillation amplitude.

It might be said that there is effected a frequency swing of the frequency F in a range between F and F that is, practically between two frequencies F(1+k) and F(1k), in which k represents a value distinctly less than /2. This swing is effected in a time T which is the period of the frequency swing.

Under these conditions, a system of complex waves is obtained which, strictly speaking, is not a standing wave system but a system which has interference fringes which for convenience sake may be designated by the terms antinode and node according, as they correspond, to a amplitude maximum or minimum, it. being however understood that these antinodes, for example, are mobiles owing to the fact that the reflected waves have frequencies which differ from those of the incident waves and have amplitudes which are also variable as a result of the damping of the waves in the ingot.

This analogy is the more acceptable as, when the frequency varies at a slow rate the difference between the frequencies of the reflected waves and the incident waves is small and the velocity of displacement of the inter ference fringes proportional to this distance-is therefore low.

The mathematical determination of the frequency swing and of the period of this frequency swing is very difficult since the rate of growth of the ingot, the damping of the natural oscillations of the system and the variation in the efficiency of the oscillation generator as a function of the frequency, should be taken into account.

Tests we have carried out on bars of thorium, uranium and copper show that a modulation of i25% either side a of frequency F, that is, the nominal frequency of the generator of mechanical oscillations (sonic or supersonic oscillations), results in a regular and continuous effect of the oscillations throughout the length of the treated ingot. Such a variation moreover covers the major part of the resonance curve as shown in FIG. 1. If represents the wave length corresponding to the frequency F, the value of the frequency F must be adjusted to that of the variable frequency F each time the length of the ingot has increase by M2. If t is the time interval during which the ingot grows to the extent of M2, experience has shown that about ten periods of frequency variation for each time interval is suflicient.

For a better understanding of the nature and objects of the invention, reference should be made to the following description and drawing, in which:

FIG. 1 is the curve hereinabove referred to;

FIG. 2 is a diagrammatic view of an apparatus for carrying out the method of the invention.

Referring to the drawing in PEG. 2, the metal ingot which is being continuously cast and processed, shown at 1, is rigidly connected, through a coupling medium 2, to a coupling steel bar 3 called piston; this assembly is maintained vertically, the lower face of the steel piston being rigidly fixed to the laminated nickel bars 4 of an oscillator generator of the magnetostrictive type further comprising a magnet (not shown) and energizing windings 5 fed from a sonic or supersonic frequen y current generator 6.

The coupling between the various materials ll234 must be as perfect as possible. Further it can be provcn that, in order to avoid reflections, it is advantageous to couple two successive media of acoustic impedances a and b by means of an intermediary material WhOSe acoustic impedance is equal to Vab, the acoustic impedance being the product DV of the density D of the medium and the propagation velocity V in said medium of longitudinal Waves generated by the vibration.

In the case of a nickel-steel junction such :as the junction of elements 3 and 4, the acoustic impedances are sulficiently close to each other (a=4.93 and b::4.76) not to require an intermediary coupling medium; moreover, a perfect coupling can be achieved by brazing or screwing. Such is not the case of the junction between steel 3 and the metal 1 to be processed when said metal has an acoustic impedance quite different from that of steel. For example, this is the case if the metal 1 is thorium (12:3.2); the intermediary impedance is then 3.9. Then, the intermediary coupling element 2 may consist of a thin pellet of annealed palladium which is highly suitable for the purpose. This palladium pellet may be pressed between the outermost faces of the steel member 3 and of the ingot 1 which are prearranged and are clamped together by any clamping means (not shown), the transmission of the vibrations between element 1 and 3 being effected through the palladium pellet 2.

The upper face of the ingot is fed through a consumable electrode 7 which is heated by heating means (not shown), and has a continuous downward vertical motion imparted thereto so that the metal thereof feeds by fusion the ingot being formed. The solidification of the ingot 1 is achieved in an ingot mold 8 which has a cylindrical shape having a vertical axis and is watercooled through a coil of metal tubing 9 connected to flexible water-hoses 10.

The ingot and the ingot mold are vertically movable relative to each other, whereby the ingot gradually solidifies. In the shown embodiment, it is the ingot mold 8 which is raised. The entire unit is housed in a furnace casing 11 sealed by means of flexible joints 12.

While the molten upper porti n of the ingot is solidifying, it is subjectedto the elastic waves generated by the magnetostriotive generators 4*S6. The frequency F of these waves is roughly adjusted so as to be approximately equal to the natural frequency F0 of the ingot, whereby, when this equalization occurs, a maximum of vibration, or antinode, appears at the free end of the ingot, in space coincidence with the molten metal. Due to the fact that in practice such a coincidence or tuning is hardly possible, the frequency F is modulated, the frequency swing being equal to about :25% of the frequency F.

Moreover, the frequency F is stepWise, manually readjusted every time that the length of the ingot increases by M2, A being the wavelength of the elastic wave in the ingot.

For these purposes the sonic or supersonic frequency current generator 6 may comprise an oscillator 13 which can be tuned to generate an adjustable frequency within a range comprising the extreme values of the natural frequency F0, corresponding respectively to the extreme lengths of the ingot at the beginning and at the end of the casting operation, a frequency selector 14 for adjusting the value of F to equalize, at least roughly, every time that this adjustment is necessary, said frequence F with the value of F0 at that time by means of a tuning knob 15 and an index 16; and a frequency modulator 17 which modulates the frequency F thus selected, the frequency swing being about i25%.

All these apparatus belong to the well-known prior art and need no description.

This method has proved advantageous in that it results, for a given degree of refinement of the metal, in a large increase in the efficiency in the art of refining metal, the possible rate of production of refined metal of this method being about ten times that of the known method.

Having now described our invention, what we claim as new and desire to secure by Letters Patent is:

1. A process of simultaneously casting and vibrating an ingot of metal for the purpose of refining the grain of said metal, comprising the simultaneous application of the following treatments:

(a) maintaining a solid ingot of said metal in a vertical position in an ingot mold;

(b) supplying the upper end of said ingot with molten metal;

(c) mechanically vibrating a bar of magnetostrictive metal and transmitting the vibrations to said ingot solely through intermediate rigid elements rigidly connected together;

(d) continuously modulating the frequency F of the vibrations, whereby the frequency value is between values F(l-k) and F(l+k), k being a value less than /2;

(e) step-wise controlling the value of the frequency F every time that the ingot length has increased by 12kt, being the wavelength of the elastic wave in the ingot, to equalize, at least roughly, said frequency F with the natural frequency P0 of the ingot, at that time, whereby the value Fo/F is constantly maintained between two limits sufficiently close to unity and to each other to correspond to relatively high values of the oscillation amplitude;

(f) permanently cooling the solid portion of the ingot to solidify the molten metal while said ingot is being vibrated, said solid portion acting constantly as a support for the supply of molten metal;

(g) and continuously displacing said ingot mold and said ingot relatively to each other, thereby constantly maintaining in said ingot mold the portion of the ingot in the process of solidification.

2. A process as claimed in claim 1, wherein the period of the frequency swing is of the order of t/n, t being the time in which the length of the ingot increases by Zkk, n being a number comprised between 5 and 20.

3. A process of simultaneously casting and vibrating an ingot of metal for the purpose of refining the grain of said metal, comprising the simultaneous application of the following treatments:

(a) maintaining a solid ingot of said metal in a vertical PQS EiQIl it an ingot mold;

(b) supplying the upper end of said ingot with molten metal;

(c) mechanically vibrating a bar of magnetostrictive metal and transmitting the vibrations to said ingot (d) continuously modulating the frequency F of the vibrations whereby the frequency value is between values F(lk) and F(l+k), k being approximately equal to A;

solely through intermediate rigid elements rigidly 5 (e) step-wise controlling the value of the frequency F connected together, the transmission from an element every time that the ingot length has increased by having an acoustic impedance a to the ingot having 2k)t, being the wavelength of the elastic Wave in the an acoustic impedance [2 occurring through an interingot, to equalize, at least roughly, said freznediate element having an acoustic impedance in; quelley F i h natural frequency F0 f (d) continuously modulating the frequency F of the 10 the ingot, at f fl whereby the Value FO/F 1S vibrations, whereby the frequency value is between Constantly malntelPed between /4 and /4, to values F (l-k) and F( 1+k), k being a alue l correspond to relatively high values of the oscillatlon than 1 amplitude;

(e) step-Wise controlling the value of the frequency F P ma y Cooling the S01id P l Q of e ingot every time that the ingot length has increased by to solldify the molten metal while said ingot is being MA, A being the wavelength of the elastic wave in the vibrated, 831d sohd Pomon aetmg constantly as 3 pingot, to equalize, at least roughly, said frequency F P011forthesupplyoflpoltel} l; with the natural frequency P0 of the ingot, at that f f ously displacing said ingot mold and time, whereby the value Fo/F is constantly mainsaid ingot relatively to each other, thereby constantly tained between two limits sufficiently close to unity malntammg 1n Sald lngol mold the POTUOD 0f llhe and to each other to correspond to relatively high values of the oscillation amplitude;

(f) permanently cooling the solid portion of the ingot to solidify the molten metal while said ingot is being vibrated, said solid portion acting constantly as a support for the supply of molten metal;

(g) and continuously displacing said ingot mold and ingot in the process of solidification 5. A process as claimed in claim 4, wherein the period of the frequency swing is of the order of t/ 10, 2 being the time in which the length of the ingot increases by )\/2.

References Cited UNITED STATES PATENTS said ingot relatively to each other, thereby constantly 2 848 775 8/1958 Ettenreich X maintaining in said ingot mold the portion of the 3193889 7 /1965 Lane et a1 164 48 ingot in the process of solidification. l u 4. A process of simultaneously casting and vibrating FOREIGN PATENTS; an ingot of metal for the purpose of refining the grain of 1,263,935 5/1961 France.

said metal, comprising the simultaneous application of the following treatments:

(a) maintaining a solid ingot of said metal in a vertical position in an ingot mold;

(b) supplying the upper end of said ingot with molten metal;

(0) mechanically vibrating a bar of magnetostrictive metal and transmitting the vibrations to said ingot solely through intermediate rigid elements rigidly connected together;

OTHER REFERENCES J. SPENCER OVERHOLSER, Primary Examiner. Y. K. RISING, Assistant Examiner, 

1. A PROCESS OF SIMULTANEOUSLY CASTING AND VIBRATING AN INGOT OF METAL FOR THE PURPOSE OF REFINING THE GRAIN OF SAID METAL, COMPRISING THE SIMULTANEOUS APPLICATION OF THE FOLLOWING TREATMENTS: (A) MAINTAINING A SOLID INGOT OF SAID METAL IN A VERTICAL POSITION IN AN INGOT MOLD; (B) SUPPLYING THE UPPER END OF SAID INGOT WITH MOLTEN METAL; (C) MECHANICALLY VIBRATING A BAR OF MAGNETOSTRICTIVE METAL AND TRANSMITTING THE VIBRATIONS TO SAID INGOT SOLELY THROUGH INTERMEDIATE RIGID ELEMENTS RIGIDLY CONNECTED TOGETHER; (D) CONTINOUSLY MODULATING THE FREQUENCY F OF THE VIBRATIONS, WHEREBY THE FREQUENCY VALUE IS BETWEEN VALUES F(1-K) AND F(1+K), K BEING A VALUE LESS THAN 1/2; (E) STEP-WISE CONTROLLING THE VALUE OF THE FREQUENCY F EVERY TIME THAT THE INGOT LENGTH HAS INCREASED BY 2K, BEING THE WAVELENGTH OF THE ELASTIC WAVE IN THE INGOT, TO EQUALIZE, AT LEAST ROUGHLY, SAID FREQUENCY F WITH THE NATURAL FREQUENCY FO OF THE INGOT, AT THAT TIME, WHEREBY THE VALUE FO/F IS CONSTANTLY MAINTAINED BETWEEN TWO LIMITS SUFFICIENTLY CLOSE TO UNITY AND TO EACH OTHER TO CORRESPOND TO RELATIVELY HIGH VALUES OF THE OSCILLATION AMPILTUDE; (F) PERMANENTLY COOLING THE SOLID PORTION OF THE INGOT TO SOLIDIFY THE MOLTEN METAL WHILE SAID INGOT IS BEING VIBRATED, SAID SOLID PORTION ACTING CONSTANTLY AS A SUPPORT FOR THE SUPPLY OF MOLTEN METAL; (G) AND CONTINUOUSLY DISPLACING SAID INGOT MOLD AND SAID INGOT RELATIVELY TO EACH OTHER, THEREBY CONSTANTLY MAINTAINING IN SAID INGOT MOLD THE PORTION OF THE INGOT IN THE PROCESS OF SOLIDIFICATION. 